1. Field of the Invention
The present invention relates to a light-emitting body including an auxiliary electrode for feeding an assist voltage, a light-emitting device, and a light-emitting display.
2. Description of the Background Art
Light-emitting bodies for use in a display are generally classified into field emission devices and EL (Electro Luminescent) devices. An EL device is implemented as an organic EL device whose light-emitting layer is formed of an organic material or as an inorganic EL device whose light emitting layer is formed of an inorganic material.
An organic EL device is made up of an anode, a cathode, and an ultrathin-film organic EL layer intervening between the anode and the cathode and formed of an organic light-emitting compound. When a voltage is applied between the anode and the cathode, holes and electrons are respectively injected from the anode and cathode into the organic EL layer and recombined therein. The resulting energy excites the molecules of the organic compound constituting the organic EL layer. The EL layer emits light when the excited molecules are deactivated to the ground level.
More specifically, the organic EL layer includes at least one of three different organic layers generally referred to as an emission layer, a hole transport layer, and an electrode transport layer, respectively, in the form of a single layer or a laminate. The emission layer emits light due to the recombination of holes and electrons. The hole transport layer allows holes to be easily injected therein, but obstructs the migration of electrons. Conversely, the electron transport layer allows electrons to be easily injected therein, but obstructs the migration of holes.
An organic EL device now attracting increasing attention includes a transparent electrode (hole injection electrode or anode) formed of, e.g., indium tin oxide (ITO). Triphenyldiamine (TPD) or similar hole injecting material is deposited on the transparent electrode by evaporation, forming a thin film. An aluminum quinolinole complex (Alq3) or similar fluorescent material is stacked on the above thin film to form an emission layer. Further, a metal electrode with a small work function (electron injection electrode or cathode) implemented by, e.g., AgMg is stacked on the emission layer. This kind of organic EL device attains luminance as high as several ten thousand candelas for a square millimeter for a low voltage around 10 V and is expected to desirably implement electric parts and displays for household appliances and vehicles.
More specifically, the organic layers included in the organic EL device each are sandwiched between a scanning (common line) electrode and a data (segment line) electrode and formed on the transparent substrate, e.g., glass substrate. The scanning electrode and data electrode play the role of the electron injection electrode and hole injection electrode, respectively. Displays using such organic EL devices are classified into a matrix display and a segment display. A matrix display causes scanning electrodes and data electrodes arranged in a matrix to display dots (pixels) for thereby displaying, e.g., an image or characters in the form of a dot matrix. A segment display displays, e.g., an image with independent display segments identical in size and configuration.
The segment display is capable of driving the display segments one by one with a static drive system. In this respect, the matrix display uses a dynamic drive system for driving the scanning lines and data lines by time division.
A first type of light-emitting body constituting the light-emitting portion of an organic EL device is implemented as a laminate of a transparent substrate, a transparent electrode, a light-emitting material layer, and a metal electrode. In this type of light-emitting body, light issuing from the emission layer is sequentially transmitted through the transparent electrode and transparent substrate in this order. A second type of light-emitting body is a laminate of a substrate, metal electrode, a light-emitting material layer, and a transparent layer. In this type of light-emitting body, light issuing from the emission layer is transmitted through the transparent electrode and then output via a film opposite to the substrate. The first type of light-emitting body is disclosed in, e.g., C. W. Tang et al. “Organic electroluminescent diodes”, Appl. Phys. Lett., 51 (12), 21 Sep. 1987, pp. 913-915. The second type of light-emitting body is disclosed in, e.g., D. R. Gaigent et al. “Conjugated polymer light-emitting diodes on silicon substrates”, Appl. Phys. Lett., 65 (21), 21 Nov. 1994, pp. 2636-2638.
It has been customary with the organic EL device to use natural injection based on a difference between the work function of the anode and that of the light-emitting material layer (hole injection) or between the work function of the cathode and that of the light-emitting material layer (electron injection). More specifically, holes are injected from the material with a great work function into the material with a small work function while electrons are injected from the latter into the former. In this sense, the anode should preferably be formed of a material with a work function as great as possible relative to the emission layer while the cathode should preferably be formed of a material with a work function as small as possible.
A typical light-emitting device outputs light issuing from the light-emitting material via either one of the anode and cathode. However, from the transparency and conductivity standpoint, materials applicable to the transparent electrode at the present stage of development are limited to ITO, an In oxide and Zn oxide mixture and so forth having a work function as small as about 4 eV. Therefore, when the transparent electrode is used for the cathode, holes cannot be injected from the transparent electrode into the light-emitting material layer unless the work function of the latter is smaller than the work function of the former. As a result, the cathode for injecting electrons in the light-emitting material layer must, of course, be formed of a material with an even smaller work function.
A small work function, however, means small Fermi-level energy and therefore causes atoms to lose electrons when subjected to a low voltage. Such electrons can therefore be easily combined with other atoms, i.e. highly reactive. Consequently, if oxygen and hydrogen, for example, exist around the cathode material, then chemical reaction occurs therebetween, producing an oxide of the cathode material or similar compound. This compound obstructs the migration of electrons from the cathode to the light-emitting material layer and thereby lowers luminance.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 06-045074, 08-264278, 09-035871, 63-264692, 08-012600, 63-295695, 05-234681, 05-239455, 08-012969, 03-255190, 05-070733, 06-215874, 02-191694, 03-000792, 05-299174, 07-126225, 07-126226, 08-100172 and 05-258859 as well as in EP 0 650 955 A1.